Cooperative cluster-Jahn-Teller effect as a possible route to antiferroelectricity

TL;DR

This study reveals an antipolar phase in cubic GaNb4S8 driven by a cooperative Jahn-Teller effect, suggesting a potential pathway to antiferroelectricity through structural and orbital ordering transitions.

Contribution

It demonstrates the role of cooperative Jahn-Teller effects in inducing antipolar phases in lacunar spinels, offering new insights into their structural and electronic phase transitions.

Findings

Observation of antipolar phase in GaNb4S8 at 31 K

Evidence of local symmetry lowering preceding orbital order

Potential to transform into a ferroelectric state

Abstract

We report the observation of an antipolar phase in cubic GaNb$_4$S$_8$ driven by an unconventional microscopic mechanism, the cooperative Jahn-Teller effect of Nb$_4$S$_4$ molecular clusters. The assignment of the antipolar nature is based on sudden changes in the crystal structure and a strong drop of the dielectric constant at $T_\mathrm{JT}=31$\,K, also indicating the first-order nature of the transition. In addition, we found that local symmetry lowering precedes long-range orbital ordering, implying the presence of a dynamic Jahn-Teller effect in the cubic phase above $T_\mathrm{JT}$. Based on the variety of structural polymorphs reported in lacunar spinels, also including ferroelectric phases, we argue that GaNb$_4$S$_8$ may be transformable to a ferroelectric state, which would further classify the observed antipolar phase as antiferrolectric.